This invention pertains to the field of antennas and more particularly, to a meander line loaded antenna configured as a small, tunable patch antenna.
Microstrip patch antennas are known in the art, and generally have a dielectric substrate with a ground plane on one surface and a strip conductor feeding a large patch. The patch is generally large, with the length being a little less than half a wavelength at the operating frequency and the width selected for the appropriate radiating resistance.
Suspended patch antennas are also known in the art, wherein the patch is suspended substantially parallel and above the ground plane. The suspended design has an increased efficiency, but is otherwise restrained with the limitations of the standard patch antenna, namely being relatively large.
In the prior art, efficient antennas have typically required structures with minimum dimensions on the order of a quarter wavelength of their intended radiating frequency. These dimensions allowed the antennas to be easily excited and to be operated at or near their resonance, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. These antennas tended to be large in size at their resonant wavelengths. Further, as the operating frequency decreased, the antenna""s dimensions were increased proportionally.
In order to address the shortcomings of traditional antenna design and functionality, the meander line loaded antenna (MLA) was developed. The basic theory and design of the MLA is presented in U.S. Pat. No. 5,790,080. An example of a basic MLA, also known as a variable impedance transmission line (VITL) antenna, is shown in FIG. 1, generally at reference number 100. The antenna 100 consists of two vertical sections (i.e., plates) 102 and a horizontal section 104. The vertical and horizontal sections 102, 104, respectively, are separated by gaps 106. Also part of the antenna 100 are meander lines 200 (FIG. 2), which are typically connected between at least one of the vertical sections 102 and the horizontal section 104 at the gaps 106.
The meander line 200 is designed to adjust the electrical (i.e., resonant) length of the antenna 100. In addition, it is possible to switch lengths of the meander line 200 in or out of the circuit quickly and with negligible loss in order to change the effective electrical length of the antenna 100. This switching is possible because the active switching devices (not shown) are usually located in the high impedance sections of the meander line 200. This keeps the current through the switching devices (not shown) low and results in very low dissipation losses in the switches, thereby maintaining high antenna efficiency. Switching of sections of a meander line using microelectromechanical systems (MEMS) switches or the like are well known to those skilled in the antenna design arts.
The basic antenna of FIG. 1 can be operated in a loop mode that provides a 360xc2x0 coverage (i.e., radiation) pattern. Horizontal polarization, loop mode, is obtained when the antenna is operated at a frequency such that the electrical length of the entire line including the meander lines 200 is a multiple of full wavelength, as shown in FIG. 3C. The antenna can also be operated in a vertically polarized, monopole mode by adjusting the electrical length to an odd multiple of a half wavelength at the operating frequency, FIGS. 3B and 3D, respectively. The meander lines 200 can be tuned using electrical or mechanical switches (not shown) to change the mode of operation at a given frequency or to switch frequencies using a given mode.
The invention of the meander line loaded antenna allowed the physical antenna dimensions to be significantly reduced in size while maintaining electrical lengths that were still multiples of a quarter wavelength. Antennas and radiating structures built using this design approach operate in the region where the limitation on their fundamental performance is governed by the Chu-Harrington relation:
Efficiency=FV2Q
where:
Q=Quality Factor;
V2=Volume of the structure in cubic wavelengths; and
F=Geometric Form Factor (F=64 for a cube or a sphere)
Meander line loaded antennas achieve the efficiency limit of the Chu-Harrington relation while allowing the antenna size to be much smaller than a wavelength at the frequency of operation. Height reductions of 10 to lower quarter wave monopole antennas are achieved while realizing comparable gain.
The existing MLA antennas are narrow band antennas. While the patch antennas have a greater bandwidth, they are too large to be useful in certain size constrained applications.
U.S. Pat. No. 5,790,080 entitled MEANDER LINE LOADED ANTENNA, describes an antenna that includes one or more conductive elements for acting as radiating antenna elements. Also provided is a slow wave meander line adapted to couple electrical signals between the conductive elements, wherein the meander line has an effective electrical length that affects the electrical length and operating characteristics of the antenna. The electrical length and operating mode of the antenna may be readily controlled.
A tunable microstrip patch antenna is described in U.S. Pat. No. 5,777,581. The patch is configured with numerous switchable microstrips. The resonant frequency of the patch is inversely proportional to the total effective patch length including the microstrip sections. Switching the microstrips changes the properties of the antenna allowing the resonant frequency to be manipulated. Other tunable microstrip patch antennas include U.S. Pat. No. 6,005,519 and U.S. Pat. No. 4,821,041.
U.S. Pat. No. 6,034,637 describes a double resonant wideband patch antenna that includes a planar resonator forming a substantially trapezoidal shape having a non-parallel edge for providing a substantially wide bandwidth. A feed line extends parallel to the non-parallel edge for coupling, while a ground plane extends beneath the planar resonator for increasing radiation efficiency.
U.S. Pat. No. 6,008,762 describes a folded quarter-wave patch antenna that includes a conductor plate having first and second spaced apart arms. A ground plane is separated from the conductor plate by a dielectric substrate that is approximately to the conductor plate. The ground plane is electrically connected to the first arm at one end and a signal unit is also electrically coupled to the first arm. The signal unit transmits and/or receives signals having a selected frequency band. The folded quarter-wave patch antenna can also act as a dual frequency band antenna. In dual frequency band operation, the signal unit provides the antenna with a first signal of a first frequency band and a second signal of a second frequency band.
Thus, the prior art inventions have been unable to produce small patch antennas. The small patch antennas are necessary in certain applications. What is needed is a small patch antenna with comparable characteristics and capabilities of the larger conventional patch antennas.
The antenna of the present invention differs from those of the prior art in that it includes a meander line allowing a physical length significantly smaller than its electrical length at its resonant frequency. This is accomplished by using switches to adjust the electrical length of the meander line. The invention also includes a fixed or switched capacitance to cancel meander line inductance. The net effect is that the size of the inventive antenna is significantly smaller than a conventional patch antenna at a particular operating frequency.
In accordance with the present invention there is provided a meander line loaded patch antenna where microelectromechanical systems switches or the like are used to electrically connect and disconnect sections of the meander line from the circuit. A capacitance value calculated for the resonant frequency provides compensation for inductive reactance of the meander line over a fairly wide range. Also, switching series capacitors are within the scope of the invention. By using these techniques, a quarter wave mode patch antenna may be constructed which occupies significantly less space than do conventional meander line loaded antennas. This size reduction is important when the antennas are to be used on mobile platforms where real estate is at a premium and larger antennas cannot be used.
It is therefore an object of the invention to provide a meander line loaded tunable patch antenna, comprising a pair of vertical sections disposed substantially perpendicular to a ground plane, at least one of the pair of vertical sections being electrically connected to the ground plane with a substantially horizontal top section disposed above and substantially perpendicular to the pair of vertical sections and each end of the top section being proximate one of the pair of vertical sections and electrically separated therefrom by a gap. There is at least one meander line proximate at least one of the gaps and operatively connected to one of the vertical sections and to the top section, the meander line comprising at least two portions selectively electrically connectable one to the other. A meander line switching means is disposed between the portions of the meander line for electrically connecting a first of the portions to a second of the two portions. Finally, there is at least one series capacitor operatively connected to at least one of the two portions of the meander line.
Another object is the meander line loaded tunable patch antenna, wherein the meander line switching means comprises at least one from the group: diode, relay, microelectromechanical systems (MEMS) switch.
A further object is the meander line loaded tunable patch antenna, wherein at least one series capacitor is operatively connected to at least one of the portions of the meander line and is calculated to effectively cancel a meander line inductance. The meander line loaded tunable patch antenna has physical dimensions much smaller than one quarter wavelength at a predetermined operating frequency and is tuned by selectively electrically adding and removing at least one of the portions of the meander line using the meander line switching means.
An object includes at least two series capacitors, at least one of the series capacitors being operatively connected to at least one meander line by a capacitance switching means. The capacitance switching means comprises at least one from the group: diode, relay, microelectromechanical systems (MEMS) switch. The series capacitors are calculated to effectively cancel the meander line inductance by selectively electrically adding and removing the series capacitor using the capacitance switching means.
An additional object is the meander line loaded tunable patch antenna, wherein the meander line switching means is located on a high impedance section of the meander line. And, where the meander line is affixed to a sheet of dielectric material disposed between the meander line and the horizontal top section. There can also be sections of dielectric material disposed at a junction between the pair of vertical sections and the ground, and also sections of dielectric material disposed at each gap.
An object of the invention is a meander line loaded tunable patch antenna, comprising a pair of vertical sections disposed substantially perpendicular to a ground plane, at least one of the pair of vertical sections being electrically connected to the ground plane, with a substantially horizontal top section disposed above and substantially perpendicular to the pair of vertical sections with each end of the top section being proximate one of the pair of vertical sections and electrically separated therefrom by a gap. There is at least one printed circuit meander line proximate at least one of the gaps and operatively connected to one of the vertical sections and to the top section, the meander line comprising at least two portions selectively electrically connectable one to the other with a meander line switching means disposed between the portions of the meander line for electrically connecting a first of the portions to a second of the portions. At least one series capacitor is operatively connected to at least one of the portions of the meander line. An advantage of using the printed circuit board is that the printed circuit meander line is attached directly to the horizontal top section.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein I have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me on carrying out my invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.